The production of dextran is known. Interest in dextran polymers began largely in view of their potential for use in various food products as conditioners, stabilizers, "bodying agents" or related uses in which they would replace natural gums. The term "dextran" collectively describes a large class of bacterial extracellular hydrocolloid homopolysaccharides consisting of alpha-D-glucopyranosyl units polymerized predominately in an alpha, 1.fwdarw.6 linkage. Dextran is synthesized from sucrose catalyzed by dextransucrose (sucrose: 1,6-alpha-D-glucan 6-alpha-glucosyltransferase EC 2.4.1.5). Thus, the dextransucrase enzyme catalyzes the synthesis of dextran via the reaction: sucrose.fwdarw.dextran+fructose. Hydrolysis of the sucrose provides the energy required for the condensation of D-glucopyranosyl units. The reaction can be carried out in vitro, without energy or cofactors, using only purified enzyme and sucrose.
Many different species of the genera Leuconostoc, and other bacteria, are known to synthesize dextran polymers from sucrose, e.g. Leuconostoc mesenteroides, e.g. the strain NRRL B-512 (F) or ATCC 10830. Sucrose induces the formation of dextransucrose in the organism Leuconostoc mesenteroides. It is reported by Donald E. Brown and Alexander McAvoy "A pH-Controlled Fed-Batch Process for Dextransucrose Production," J. Chem. Tech. Biotechnol. 1990, 48, 405-414, that a commercial dextran is commonly produced by the bacterium Leuconostoc mesenteroides strain NRRL B-512-F which contains 95% of 1,6- and 5% of 1,3-D glucopyranosidic linkages, and that most of the dextran is marketed as either blood volume expander or flow improver with the means of their molecular weight distributions at 70,000 and 40,000 respectively.
Certain strains of yeast, notably strains of Lipomyces starkeyi, are known to have dextranase activity. Whereas most strains of this organism have not been considered for the commercial production of dextranase because of its slow growth, and the difficulty of avoiding contamination from other organisms during growth, Lipomyces starkeyi ATCC No. 12659 has been used to hyperproduce extracellular dextranase at lesser lag time when cultured at pH ranging about 2.5 to 4.5 in an aqueous nutrient medium containing nitrogen and mineral sources, and an assimilable carbon source for growth and energy . . . viz. a dextranase inducer, suitably dextran, or glucose. This process, organism and a method for the production of this organism from a parent strain, Lipomyces starkeyi ATCC No. 20825, a less dextranase-active species per se, are described by reference to U.S. Pat. No. 4,732,854 which issued Mar. 22, 1988 to Donal F. Day et al.
The dextran that is produced by current technology is of relatively high molecular weight, and the molecular weight range distribution is quite wide. Thus, e.g., the best current technology for dextran products consists in producing dextran by fermentation with sucrose with Leuconostoc mesenteroides, separating the polymer by alcohol precipitation, conducting enzymatic or acidic hydrolysis of the polymer, and then chromatographically separating the polymer into the desired fractions. The product yields have a high polydispersity index. To obtain dextran polymers of a more select, narrower molecular weight range distribution, e.g. a polydispersity index of about 1 or 2, additional processing steps are required, e.g. enzymatic hydrolysis and chromatographic separation steps. This, of course, adds to the costs of producing polymers of preselected, narrow molecular weight range distribution as is required in the production of many dextran polymer products, e.g. blood plasma. Thus, a better process is needed for this purpose.